This invention relates to a program shutter and more particularly, to a program shutter of the type wherein the open/close action of shutter blades is performed by a stepping motor.
A program shutter of the type wherein the open/close action of shutter blades is performed by a stepping motor is configured such that upon control of a release button, a drive pulse signal having a given period is supplied to the stepping motor, the motor rotates forward to gradually open sectors, when a stepping rotation corresponding to an exposure level has taken place the phase of the drive pulse signal is changed, and the stepping motor rotates backward to close the sectors, whereby a proper exposure is obtained.
In such a shutter whose sectors are opened and closed by a stepping motor, however, since a sector operating member is restricted by a spring to a reference position in the quiescent state to prevent the actuation of the sectors, a load owing to the spring is imposed on the stepping motor in the course of sector opening, and due to the inertia of each component in motion or the fitting error of each component, the variation of aperture diameter relative to the elapse of time in the course of opening becomes uneven, resulting in a problem of no accurate exposure control being accomplished.
Therefore, to solve the foregoing problem, the present applicant has proposed in Japanese Patent Application No. 61-297909 to make a drive pulse signal for a stepping motor have a pulse width as synchronized to the operating characteristic of a shutter mechanism, which is featured in that a lag portion of the operating characteristic is compensated by elongating the pulse width. Since such log is especially apparent when the stepping motor changes from forward to backward rotation and the pulse width must always be elongated. Thus, another problem results that increasing the response time of a program shutter performing so-called triangular waveform exposure is difficult to accomplish.
Further, if a high-speed, powerful stepping motor is used to increase the speed, or to actuate shutter blades at high speed so that a very short exposure time can be attained at high speed, this provision is unfavorable from the standpoint of cost.